A usage-time control-apparatus is known, which uses a read-out device and an information carrier to determine the residence-time of a visitor in an installation by measuring the time difference between entry and exit of the visitor. The exit from the installation, normally closed, is openable in dependence on the time difference determined. Additionally a "money-return" device can be coupled to the time-difference measuring device by means of which, depending on the usage time of the installation, the unused remainder of the fee deposited on entry to the installation is refunded to the user. It is equally possible to couple an indication requesting additional money or fees to the time-difference measuring device. It has been proposed that the magnetizable data carrier be designed so as to yield magnetic information signals readable as distinct audio frequencies. Upon return of the information carrier the audio frequency read from the information carrier is compared with the audio frequency currently provided by the coding unit. An implementation of this type is disadvantageous, for the information carrier must traverse the read-out means with a constant velocity. If the velocity is altered, the result is an erroneous reading of the impressed audio frequency. Also if two magnetic regions are not equidistant and a constant coding or reading velocity is employed this will result in a change which may lead to errors or disturbances in the control apparatus. On one hand one is faced with a very sensitive and costly installation, and on the other hand this solution requires the insertion of an information carrier in a certain position. Also this type of information carrier, which has the form of a punched card, is not suitable for certain installations, such as a bathing establishment.